1. Field of the Invention
The invention relates generally to mechanical seals used to prevent fluid leakage from pumps, mixing vessels, gear boxes, and other containers containing a fluid and which are penetrated by a rotatable element. More particularly, the invention pertains to a cartridge-type mechanical seal and a back plate, adapted to replace the conventional gland and “stuffing box” seal of a fluid pump.
2. Description of the Prior Art
The oldest mechanical pump designs used a gland to prevent leakage of fluid around the rotatable pump shaft. The gland comprised a sleeve within a “stuffing box” fitted over the drive shaft for the pump's impeller. The sleeve was tightened over compressible packing material to such an extent as to prevent fluid leakage around the shaft, while allowing the shaft to rotate. This design suffered from wear and contamination of the packing material, which eventually resulted in fluid leakage around the gland.
Later designs contemplated the replacement of the packing material with a cartridge-type mechanical seal, located within the seal cavity or “stuffing box” of the pump. Prior art mechanical seals commonly included a metal cylindrical sleeve, positioned around the pump shaft. At one end of the sleeve, an O-ring extended between the shaft and a groove in the inner wall of the sleeve, providing a first seal. Mechanical seals additionally included rotary and stationary components having contacting faces to provide the primary hydraulic seal. For adequate seal performance, a relatively tight contact between the two sealing faces must be maintained at all times. For that purpose, biasing means urging the sealing faces of the rotary and stationary components together were employed. Biasing means was provided by a spring located between the end of the seal sleeve and the rotary or stationary components of the seal to be maintained in sealing relation.
Most prior art cartridge mechanical seals for centrifugal pumps are designed for installation into the above-referenced “stuffing box”. The pump “stuffing box”, adapted to retain packing material of indefinite dimensions and an adaptable configuration, makes a poor fit for mechanical seals having a definite size and configuration. It is not uncommon that minimum dimensions of 1/32″ to 1/16″ exist between seal assembly and the “stuffing box” bore. Such a tight fit inhibits even moderate circulation of pumped fluids in the vicinity of the seal assembly. Consequently, seal lubrication and cooling are inadequate, and solids within the pumped product may lodge and compact within the seal itself. If the seal fails prematurely from these effects, product contamination from exposure to air or other foreign material may occur.
In an attempt to overcome premature seal failure, various systems for flushing the “stuffing box” or the contained seal gland, have been developed. These systems include, active fluid circulation devices, circulation rings, and double mechanical seals with the addition of flushing media. These remedial systems add to the complexity and cost of the mechanical seals, without really solving the underlying problem: the location of the seal itself.
Lastly, from a maintenance perspective, locating the mechanical seal in the “stuffing box” makes maintenance or repair a difficult proposition. In order to service, replace, or repair the mechanical seal, the “stuffing box” has to be removed from the pump housing. Thus, a major disassembly and re-assembly procedure is required. In addition, the tight fit of the prior art cartridge seal within the “stuffing box” bore also makes installation and removal of the seal difficult.
However, there is no need or requirement that mechanical seals be located in a “stuffing box”. It is believed that this practice is merely a carry-over from the historical usage of the “stuffing box” to contain packing material, and one which should be abandoned by the industry.
Therefore, it is an object of the present invention to provide a cartridge-type mechanical seal construction, which removes the mechanical seal from the “stuffing box” and relocates it in a position proximate the pump impeller.
It is a further object of the construction disclosed herein, to effect installation and removal of the cartridge-type mechanical seal requiring less time and effort for disassembly and re-assembly of the pump than was previously required.
It is yet another object of the present invention to provide a mechanical seal construction providing large clearances between the pump shaft and the sealing faces for better lubrication, cooling, and self-cleaning action.
These and other object of the present invention will be described in the drawings and in the detailed description of the preferred embodiment set forth below.